venting of the fuel tank

[seagull]

My -12 is 2015 model with the fuel tank to one side.
A couple of days ago I put 19 gallons in my tank when the hangar was at 60f. Today the hangar is 95f and I opened the filler cap, sealed type, and heard a "pop" like air going out. I thought it was strange since the tank is vented.

To test it I put an air compressor hose into the filler neck sealed with a rag and gave a shot of compressed shop air. Fuel immediately came out of the overflow on the bottom of the fuselage.

I'm thinking about how the plumbing is done with the air vent at the top of the tank this would make sense fuel would come out when the tank is under pressure until the fuel in the tank is below the air vent fitting. I could see that it is possible to loose fuel in a topped off tank this way.

Does anyone have a different setup?

[PaulvS]

Be careful pressurising a fuel tank with an air compressor as it can easily be damaged. It sounds like the tank was filled to the top, in which case I would expect fuel to come out the vent pipe as it expands or is expelled by being pressurised.

[lr172]

If the vent routing is anything like the 6 or 7, vans routes the vent up about a foot above fuel level then back down. If that vent line gets fuel in it, the upper lift acts like a manometer and a small pressure will build in the tank if the cap seals well. The pressure produced is very small and won't harm anything.

[seagull]

Originally Posted by PaulvS

Be careful pressurising a fuel tank with an air compressor as it can easily be damaged. It sounds like the tank was filled to the top, in which case I would expect fuel to come out the vent pipe as it expands or is expelled by being pressurised.

I regulated the pressure to 5psi and did not hold it on, only a short burst.

[seagull]

Originally Posted by lr172

If the vent routing is anything like the 6 or 7, vans routes the vent up about a foot above fuel level then back down. If that vent line gets fuel in it, the upper lift acts like a manometer and a small pressure will build in the tank if the cap seals well. The pressure produced is very small and won't harm anything.

The vent line exits the tank and rises to the top of the fuselage with a “T” fitting that acts as an air break to prevent siphoning in case the line fills with gas.

I would think a vent placed at the highest point maybe in the filler neck would allow air to escape before fuel is pushed up the factory vent in the tank.

I realize the easy solution is don’t top the tank but in the -12 the last gallon or two is good for 1/2 hour of flying.

[Bob Y]

Originally Posted by seagull

I would think a vent placed at the highest point maybe in the filler neck would allow air to escape before fuel is pushed up the factory vent in the tank.

That’s exactly what Van’s did for the fuel-tank vent on the iS.

[seagull]

Are there any other vent lines on the iS?

I am considering capping off the tank vent and adding a vent to the filler neck. Removing the air break and running the new vent line out the bottom.

Can anyone think of a reason, besides the work involved, not to do this?

[Bob Y]

Nope; that’s the only one.

[Stuart Grant]

I recently installed the tank vent back to the tail cone in my legacy (2013) RV-12 still being assembled. In that setup Van's design ** has the vent (a hollow rivet like the static ports) placed directly behind the aluminum overflow tube so that it is (I assume) not in the air steam where there may be negative pressure. On Cessna's the tank vent is behind the strut cuff where the strut attaches to the wing. This is done (I assume) to block it from being in the air steam. Kitplanes magazine has articles on the complexities of fuel tank venting. Be careful.

**I followed the design shown on Drawing 37-08, Revision 3 Dated 12/29/13 and ordered the parts shown in the drawing. This revision came after my kit was shipped. I had to open the fuel tank for the service bulletin so I did other mods, the added vent port and the strainer inspection port. I will also do a vented fuel cap.